1. Field of the Invention
The present invention relates to a pixel circuit for use in an organic light emitting diode (OLED) display, more particularly, to a pixel circuit capable of compensating luminance discrepancy for use in the OLED display.
2. Description of the Related Art
With a rapid development of monitor types, novelty and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDA), digital cameras, and projectors. The demand for the novelty and colorful monitors has increased tremendously.
Liquid crystal display (LCD) monitors control pixel luminance by adjusting voltage drop applied on a liquid crystal layer of the liquid crystal display. Differing from liquid crystal displays (LCDs), Organic Light Emitting Displays (OLEDs) determine the pixel luminance by adjusting forward bias current flowing through an LED. With self-lighting technique without requiring additional light source electrode, OLEDs provide faster response time period than LCDs. In addition, OLEDs have the advantages of better contrast and wider visual angle. More important, OLEDs are capable of being manufactured by existing TFT-LCD process. The commonly used OLEDs utilize a low-temperature polysilicon thin film transistor (LTPS TFT) substrate or amorphous silicon (a-Si) substrate.
Please refer to FIG. 1, which shows a pixel circuit 10 of an OLED display according to a prior art. The pixel circuit 10 comprises a first transistor T1, a second transistor T2, a storage capacitor Cst, and an organic light emitting diode 12. When a scan signal voltage from a scan end SCAN to turn on the first transistor T1, the data signal voltage Vdata is fed from a data end DATA and is delivered to a gate electrode of the second transistor T2 via the first transistor T1. Whereon the second transistor T2 operating in a saturation region, a current Id is based on voltage drop between the source electrode and the gate electrode of the second transistor T2 (Vsg=Vdd−Vdata), in other words, Id=K(Vsg−Vt)2=K(Vdd−Vdata−Vt)2, where K is a constant, and Vt represents threshold voltage of the transistor T2. As the luminance of the OLED 12 is proportional to the current Id, thus is adjusted as the data signal voltage Vdata to show various grey levels. Moreover, the luminance of the OLED 12 can be shown a fixed gray level for a while because the data signal voltage Vdata is stored in the storage capacitor Cst.
Please refer to FIG. 2, which illustrates a relationship of the current Id versus display time under various threshold voltages. Currently with the conventional way for fabricating the OLED display employs Polysilicon Thin Film Transistor Circuit technology, it is possible that the second transistor T2 of each pixel circuit 10 on the OLED display may have different threshold voltage Vt. From the view of FIG. 2, obviously, the current Id of the second transistor T2 is varied as different threshold voltages. That will result in uneven luminance of the OLED display 12.
In addition, the current Id is also reduced as the voltage drop Vsg between the gate electrode and the source electrode caused by a decrease in the voltage Vdd due to load effect of line resistors. This phenomenon will degrade the display quality as well.
There is a need, therefore, for an improved OLED device and method for solving the problem of uneven luminance of the OLED panel resulting from the threshold voltage difference of the transistors, and a reduction of the conducting current flowing through the transistors attributed to a decrease in supply voltage Vdd due to line resistor.